Five teenagers are being held on suspicion of dangerous driving after four people died in a horrific crash between two cars.
Three 17-year-olds and a 68-year-old woman were killed by the collision which took place in Powys, Mid Wales, as a number of vehicles travelled in a convoy.
Five male drivers, aged 17 and 18, from other vehicles, were arrested, Dyfed Powys Police said.
The accident happened on the A470 near Storey Arms, Brecon, between Libanus and RAC bends, just before 10.15pm yesterday.
Two vehicles were involved - a black Volkswagen Golf estate and a green Volkswagen Golf. The black Golf is not believed to be part of the convoy.
Police said a 17-year-old male youth driving the green Golf and another 17-year-old male who was a passenger also died, and a 17-year-old female who was among three other passengers who were taken to hospital in a serious condition has since died.
All three fatalities in that car were from Barry, Vale of Glamorgan.
The driver and front seat passenger in the Golf estate were taken to hospital, and the passenger, 68-year-old Margaret Elizabeth Challis (nee Webber) from the Merthyr area has since died. The driver's condition is described as serious.
Superintendent Chris Curtis said: "This is a tragic incident and our thoughts are with the families at this difficult time. We are working with South Wales Police in order to provide support to the families affected and also the communities they live within. We will work with partners to ensure that the incident is fully investigated."
Dylan Jones, head teacher at Ysgol Bro Morgannwg in Barry, told the BBC that one of those killed was a sixth form pupil at the school.
A former male pupil had also died, he said, while a third pupil was seriously injured in hospital.
He said: "We had a junior eisteddfod this morning and some of the sixth form were stewarding the event when news started to come in.
"I have spoken to both parents and we are preparing for Monday so that we can offer all assistance and counselling to the pupils."
Anyone who witnessed the incident is asked to contact Dyfed Powys Police on 101.